Field of the Invention
The present invention relates to an optical module, in particular, the invention relates to the bi-directional optical module that transmits transmitting light to a single optical fiber and receives receiving light from the single optical fiber; and an optical transceiver implementing the optical module.
Related Background Art
When a transmitting optical beam and a receiving optical beam each propagate in an optical fiber, a wavelength division multiplexing (WDM) filter is used to divide the transmitting optical beam from the receiving optical beam. One type of the WDM filters stacks, on a substrate, a plurality of optical films having thicknesses equivalent to a quarter wavelength (λ/4) of the optical beam. The number of films determines the performance of the optical discrimination between the transmitting optical beam and the receiving optical beam, namely, the cut-off performance of the WDM filter. The sharp cut-off performance requests the number of stacking of the films.
An angle, by which the transmitting optical beam and/or the receiving optical beam enter the WDM filter, also determines the cut-off performance. As the incident angle of the optical beam becomes large, that is, as the incident angle deviates from the normal angle, the designed cut-off performance of the WDM filter becomes hard to be realized. Setting the incident angle close to the normal of the WDM filter, the designed cut-off performance may be obtained even the WDM filter decreases the number of the stacking of the films.
Various prior documents have disclosed a bi-directional optical module that optically couples with a single optical fiber. One of the documents has disclosed that a transmitter module and a receiver module each have a co-axial housing assembled with a housing that installs the WDM filter. The WDM filter makes an angle of 45° with respect to the optical axis of the transmitter module and that of the receiver module. Other documents also have disclosed optical modules including a lens module that covers a transmitter device and a receiver device, and divides the transmitting optical beam from the receiving optical beam within the lens module. However, the incident angle of the transmitting optical beam and/or the receiving optical beam with respect to the WDM filter in those prior documents has been set to be 45° same as those of the first prior document. Some prior documents have disclosed, in the arrangement of the lens module, that the incident angle with respect to the WDM filter is set to be an acute angle, but left various subjects including the crosstalk between the transmitting optical beam and the receiving optical beam.